Not sure of the age of the people who might "win" the cave trip...however, if there are youth, be sure to review the Boy Scout/NSS guidelines for youth on trips. No matter the age, be sure everyone is aware there are no "guided trips" in wild caving. Newbies must be prepared to think and act for themselves.
That said, it sounds like a great idea for a charity event. Who knows? You might even attract some new blood to your grotto!